It is known to provide a multilayer coating film comprised of an aqueous coating composition by a method in which, e.g., multiple layers of a silver halide emulsion having gelatin as a binder are simultaneously applied on a continuously moving support by means of a slide hopper type coater or an extrusion hopper type coater. Immediately after coating, the multilayers are coagulated in a cooling zone utilizing the sol to gel change phenomenon of a hydrophilic colloid such as gelatin so that the viscosity of the multilayers becomes extremely high, e.g., in the range of from 1.times.10.sup.4 to 1.times.10.sup.5 centipoise (cP). In this state, the layers hardly mix with each other. Thereafter, the temperature of the system is gradually raised to bring about drying of the multilayers, usually with hot air to facilitate evaporation of the solvent or the like. The result is a coating film comprised of multiple layers.
On the other hand, in the case where an organic type coating composition (a composition containing an organic solvent) is merely applied in multilayers and then dried, diffusion and/or mixing is apt to occur in the bead portion being coated and in the freshly coated multilayers between the time of application and the time of drying. Diffusion and/or mixing also is apt to occur in between the coated layers and the underlayers since the surface tension of organic coating compositions is low compared to the surface tension of an aqueous type coating composition, and further, since there is no sol to gel conversion step, diffusion/mixing is liable to occur during the step of drying. That is, in a coating composition comprising an organic solvent, there are no sol to gel type conversion materials having compatibility with a broad range of ingredients which can be used in the organic solvent the way in which gelatin can be used in an aqueous solvent.
Accordingly, it is very difficult to obtain a coating film in a state in which layers thereof remain fully discriminated from each other, particularly in the case of using an organic type coating composition.
For the foregoing reasons, in the case of forming a multilayer coating film comprising an organic solvent, a method in which layers are successively applied and dried one after another has been generally used. As such a successive application and drying system, there are known methods in which layers are sequentially applied and dried, methods in which a plurality of application and drying portions are provided so that application and drying are continuously performed, and so on. In the former method, however, an extremely long manufacturing time is involved so that the manufacturing cost becomes extremely large. In the latter method, on the other hand, the number of application and drying stages corresponds to the number of layers, so that the provision of extremely expensive manufacturing equipment is required and the manufacturing cost becomes extremely large.
As described above, various methods for obtaining a multilayer coating film have been proposed until now. Of those methods, the method in which a coating film is applied and dried layer by layer requires extremely large-scale equipment. The methods in which a bead is formed by a multilayer slide die or in which a curtain film is formed by a multilayer die so as to form a simultaneously multilayer coated film are not effectively used for the coating of a composition comprising an organic solvent, although the method can be effectively used for coating a composition such as a photosensitive material or the like by taking advantage of the sol to gel conversion.
The present inventors have made investigations in order to solve problems as described above, and have discovered a process for forming a multilayer coating film which is disclosed in Japanese Patent Publication No. Sho-62-51670. In this method, an electron-beam hardenable resin is added to a non-aqueous coating composition (a coating composition comprising an organic solvent). Thereby, it becomes possible to realize formation of a multilayer film continuously by multilayer application of a coating composition comprising a non-aqueous solution, a result which has been very difficult to obtain by the conventional methods. The method, however, has the technical limitation that resin to be hardened by electron beams must be contained in the coating composition, thus the coating composition is increased in viscosity, and so on.
In the art of making photosensitive printing plate precursors, most photosensitive printing plate precursors have been of the single layer type and have been produced by a method such as wheeler coating, roll coating, bar coating, bead coating, or the like.
On the other hand, various photosensitive printing plate precursors having a multilayer configuration have been disclosed, for example, in JP-B-53-36364, JP-B-50-7481, (the term "JP-B" as used herein means an "examined Japanese patent publication") JP-A-50-133008 (the term "JP-A" as used herein means an "unexamined Japanese patent publication"), DAS 1,671,626, and the like. In producing such printing plate precursors, methods such as wheeler coating, roll coating, gravure bar coating, bead coating, or the like have been used so that a lower layer is first applied and dried, and then an upper layer is applied and dried to thereby produce a photosensitive printing plate precursor having multilayers.
However, these methods of making a multilayer photosensitive printing plate precursor are deficient with respect to the following points. That is, in the process for wheeler coating, there was a problem in that since a thick film is spread by a centrifugal force, a lower layer is apt to be dissolved in the upper layer liquid in the spreading step particularly in the case where the kind of solvent in the upper layer is the same as that in the lower layer, and therefore it is difficult to obtain a film having a configuration in which upper and lower layers remain fully separated from each other.
On the other hand, in the process for roll coating, gravure coating, bar coating, or the like, there is a problem in that since coating is performed by bringing a coating roll, a gravure roll, a bar, or the like, into contact with an already-coated film surface, there is a possibility that when coating of the upper and lower layers is performed with the same group solvent, a lower layer surface is dissolved by the solvent in the upper coating layer. In an extreme case, the underlayer may even be physically damaged. The range of suitability of this type of coating is therefore extremely limited. Further, in the process for bead coating an upper layer onto an underlayer, when the same type of solvent is used for the upper and lower layers (i.e., hydrophobic or hydrophilic), the lower layer is inevitably swelled or dissolved in between coating and drying. Therefore, even this method was not always satisfactory.
Conventionally, attempts to form a coating film with maintenance of separation between multilayers have involved using solvents for the respective layers which are hardly mutually dissolved, and it has been difficult to form a multilayer film using solvents capable of being mutually dissolved and by means of roll coating, gravure coating, bar coating, bead coating, or the like.